Method for producing phosphoric acid

ABSTRACT

The invention concerns a method for producing phosphoric acid, which consists in: at least an attack of phosphate ore with an attacking liquid containing soluble phosphate ions, forming of an attacking product, separating in the attacking product between an insoluble solid phase containing impurities and a liquid phase having soluble phosphate ions and calcium ions, introducing in the separated liquid phase an acid stronger than phosphoric acid and which forms, with said calcium ions, a soluble calcium salt, and isolating a solution of said calcium salt, not contaminated by the impurities, thereby obtaining a phosphoric acid solution.

[0001] The present invention relates to a method of producing phosphoricacid starting from phosphate ores.

[0002] The production of phosphoric acid, if synthesis is excluded,takes place today principally by two methods: either by the “dry”method, which consumes a great deal of energy, in high-temperature“ore-coke-silica” furnaces, followed by bubbling through of the P₂O₅which is extracted in H₂O, or by the “wet” method of attacking withacid, principally sulphuric acid, but also nitric, hydrochloric orperchloric acids, of preconditioned (calcination, grinding, sieving)phosphate ores, with the obtaining of a raw phosphoric acid solution.The Ca salts formed, following the attack acid, are eliminated by thecrystallisation of the sulphates (from 80° C. to 110° C.), or nitrates(at a temperature of <−5° C. and low filterability) or by carrying out aliquid/liquid extraction of the phosphoric acid by separating it fromthe calcium chlorides.

[0003] The resulting product of these attacks with acid contains all theimpurities already existing in the extracted ore. These impurities thencontaminate not only the phosphoric acid produced but also the saltscrystallised or separated by extraction. This contamination is such thatit makes these salts difficult if not impossible to make use of. Thereexists at the present time a real problem in the exploitation of thegypsum formed during the sulphuric attack. It is necessary to know thatthe production of 1 tonne of H₃PO₄ expressed as P₂O₅ involves theproduction or more or less 5 tonnes of gypsum in a form which isdifficult to exploit at the present time.

[0004] Moreover, these attacks all require expensive preconditioning. Afine grinding of the extracted ore is for example absolutely necessaryif it is wished to obtain a correct yield by sulphuric attack.

[0005] The majority of these methods involve high-temperature reactionswhich require expensive cooling. The temperature in the liquid-solidattack reactions are difficult to control and these high-temperatureattack environments are very detrimental to the equipment.

[0006] Various methods are also known comprising an attack on the ore byphosphoric acid or a mixture of several acids, one of which isphosphoric acid (FR-A-2.343.696, U.S. Pat. No. 3,323,864, FR-1.082.404and EP-A-0 087 323). These methods all involve, after this attack,precipitation of a calcium salt in the product obtained by means of anacid. In some of these methods the precipitated calcium salt stillcontains the original impurities in the ore, in others the acid used orone of the acids used is hydrofluoric acid or fluosilicic acid, whichare expensive acids, which it is necessary to recycle and which aredangerous to handle given their toxicity.

[0007] The object of the present invention is a method of producingphosphoric acid making it possible to exploit the by-products to themaximum possible extent, or in any case to greatly reduce the cost oftheir elimination, whilst proposing a phosphoric acid yield equivalentto or greater than those provided by the known methods. Another purposeof the method is to produce a phosphoric acid suitable for producingfertilisers and for industrial applications, also with a view toobtaining a purified phosphoric acid.

[0008] Advantageously, this method must make it possible to avoidgrinding or calcination of the extracted ore before the attack. Themethod according to the invention will preferably be able to beimplemented at a temperature which is easily controllable and underconditions which allow reactions which do not greatly attack theequipment used, and control of the apportioning.

[0009] These problems have been resolved according to the invention by amethod of producing phosphoric acid comprising

[0010] at least one attack on phosphate ore by an attack liquidcontaining phosphate ions in solution, with the formation of an attackproduct, and

[0011] a separation in the attack product between an insoluble solidphase containing impurities and a liquid phase having phosphate ions andcalcium ions in solution,

[0012] characterised in that it also comprises

[0013] introduction into the separated liquid phase of an acid which isstronger than the phosphoric acid and which forms, with theaforementioned calcium ions, a soluble calcium salt, and

[0014] isolation of a solution of this calcium salt, not contaminated bythe impurities, with the obtaining of a solution of phosphoric acid.

[0015] The acid can for example be in this case hydrochloric acid andthere is then obtained, as the salt, calcium chloride which is notcontaminated by the impurities of the ore which is an exploitableproduct, for example as a spreading product for icy roads. It is alsopossible to envisage, as an acid of this type, perchloric acid, nitricacid etc. This method has the great advantage of an attack on theunground ore by a weak acid and elimination of the impurities normallyencountered in phosphate ores as soon as the attack by an acid ends,phosphoric acid in this case. These impurities are for example Fe, Si,U, Cd, As, F, etc; they are only a little dissolved during thephosphoric attack on the ore according to the invention. only theCa⁺⁺ion, whose content is relatively high in the mineral (up to 50%), isentrained in solution in the presence of the phosphate ions, forming inparticular at a temperature close to the ambient temperature of the Cadihydrogenophosphate. This allows, at the end of the method according tothe invention, separation between a pure calcium salt and a phosphoricacid solution, the calcium-based by-product thus being able to be muchmore easily exploited given its high degree of purity. Theaforementioned impurities are finally in the form of a concentratedsolid residue, easily storable, because of its reduced volume.

[0016] In addition, fine grinding of the ore does not prove necessaryprior to the attack. According to the invention, the calciumdihydrogenophosphate being soluble, the ore attack reaction continues asfar as the grains thereof, which significantly increases the reactionspeed.

[0017] This attack by phosphoric acid thus makes it possible to transferapproximately 78% of the P₂O₅ present in the ore into Cadihydrogenophosphate, which is a very good yield. This can be improvedstill further, up to approximately 95-98%, by a second attack by H₃PO₄on the cake of impurities, after separation by sedimentation and/orfiltration.

[0018] Liquid attack containing phosphate ions in solution means asolution or suspension, preferably aqueous, containing phosphoric acidand/or derivatives of phosphoric acid which in the dissolved stategenerate phosphate ions. The solution or suspension may also containother ions in solution or have a fraction of insolubles in suspension.

[0019] Advantageously, the attack liquid according to the invention isan aqueous solution of phosphoric acid which preferably has a P₂O₅content of 20-35%, advantageously 22% to 32%, in particular 30%.

[0020] The attack on the ore and the separation of the liquid phasecontaining phosphate and calcium ions in solution takes placeadvantageously at a temperature from 10° to 70° C., preferably from 25°to 45° C., in particular approximately 25° C.

[0021] According to an advantageous embodiment of the invention, theseparation is effected by settling of the attack product.Preferentially, the separated insoluble solid phase is formed from asolid phase with coarse grains which sediments, the separated liquidphase having in solution phosphate ions and calcium ions comprises aninsoluble solid phase suspension with fine grains, and the method alsocomprises an elimination of the fine grains in suspension byclarification of the separated liquid phase, before the aforementionedstep of introduction of the acid. This embodiment allows rapid,inexpensive and extremely thorough separation of the solid phasecontaining the impurities.

[0022] The operations of separation of the solid remainders composed ofunattacked and sterile grains (heavy metals) are carried out by theconventional ore processing methods: flocculation, sedimentation,filtration.

[0023] The sizing of the equipment necessary for these operations isdetermined principally by the granulometry of the remainders and theirsterile contents.

[0024] By way of indication, the compared analysis results of the orebefore treatment and the remainders resulting from the settling and/orfiltration give the following proportions (in mg/kg) of certain heavymetals. Ore before Remainder with Remainder with treatment fine grainscoarse grains Cr 140 ppm 330 ppm 180 ppm Al  0.5%  1.54%  1.18% Fe 0.33% 1.96% 0.711% F  3.5% 16.39%  6.99% Si  2.5%  4.55%  12.8%

[0025] According to one improved embodiment of the invention, the methodcomprises at least one additional attack of the separated solid phasewith an additional attack liquid containing phosphate ions in solution,with the formation of at least one additional attack product, and anadditional separation thereof between an additional insoluble solidphase and an additional liquid phase containing phosphate ions insolution. It is thus possible to increase the yield of the method and toreduce the volume of the residues still further.

[0026] Advantageously, according to the invention, at least part of theattack liquid and/or of the additional attack liquid comprises at leastpart of the liquid phase resulting from the said separation step and/orfrom the said additional separation step and/or at least part of thephosphoric acid solution. It is thus possible to perform the attack stepby introducing into it, as fresh material, solely ore and a solvent suchas water. The attack liquid and the additional attack liquid can beformed solely from one or more recycled liquids, issuing from the methodaccording to the invention itself.

[0027] The filtrate resulting from the attack and then separation phasescontains calcium dihydrogenophosphate.

[0028] In the second phase of the method, the so-called introduction,the filtrate is treated with a strong acid so as to generate phosphoricacid from Ca dihydrogenophosphate by displacing its basic ion.

[0029] The various operations taking place in this second phase willresult in the production of a phosphoric acid, generally at lowconcentration (around 30%).

[0030] The choice of the strong acid used is determined principally bycriteria of an economic nature (availability, security of supplies,cost) and also considerations of an ecological nature relating to thedestination of the by-products (storage or use).

[0031] The choice of the acid allowing separation of the H₃PO₄ dependson the solubility in an aqueous medium of the calcium salt resultingfrom the attack on the calcium dihydrogenophosphate by this acid.

[0032] According to an improved embodiment of the invention, theisolation step comprises, in the separated liquid phase treated with theacid, a solubilisation of the phosphoric acid in an organic solvent, notmiscible with water, in order to form an organic phase containingphosphoric acid and an aqueous phase containing the Ca salt in solutionand, in the organic phase, extraction with water in order to form thesaid aqueous solution of phosphoric acid. As an organic solvent it ispossible to use any solvent suitable for a liquid-liquid extraction inwhich the phosphoric acid is soluble and the Ca salt is insoluble underthe extraction conditions. As an organic solvent, it is possibleadvantageously to envisage an aliphatic alcohol, for example n-butanol,etc.

[0033] The treatment of the hydrochloric acid advantageously takes placeat a temperature of between 10° and 45° C., advantageously between 20°and 30° C., preferably at approximately 25° C.

[0034] The treatment with hydrochloric acid can advantageously takeplace at the same time as the introduction of the organic solvent.

[0035] The organic phase issuing from this treatment contains H₃PO₄ andHCl. It is separated from the aqueous phase containing CaCl₂ in solutionand is then advantageously subjected to two successive washingoperations.

[0036] The first washing with a solution of Ca dihydrogenophosphate caneliminate the HCl. The yield from this operation depends on the P₂O₅content of the solution of Ca dihydrogenophosphate.

[0037] The organic phase resulting from this first washing operation canif necessary be purified with a solution of phosphoric acid containing30% P₂O₅, in order to remove the traces of a few residual heavy metals.

[0038] The organic phase issuing from these various purificationoperations now contains only H₃PO₄. The extraction of H₃PO₄ will becarried out by transfer into aqueous phase by the addition of water tothe organic solvent and the separation of the two liquid phases: organicsolvent/water. The optimal conditions of this separation will bedetermined by the use of the ternary diagram H₃PO₄—organic solvent—H₂Oaccording to the concentration of H₃PO₄ in the organic solvent. At theend of this separation of the two liquid phases, the organic solvent isrecycled to a prior stage of the process and H₃PO₄ in aqueous solutioncan be concentrated by distillation.

[0039] Advantageously, according to the invention, a fraction of theaqueous solution of acid is recycled in the ore attack phase. Theremaining fraction can be concentrated by evaporation in order to give aphosphoric acid, for example of standard quality, in particular with aconcentration of 54-60% of P₂O₅.

[0040] Other advantageous embodiments of the method according to theinvention are indicated in the accompanying claims.

[0041] The invention will now be described in more detail by means ofnon-limiting example embodiments.

EXAMPLE 1

[0042] Phosphoric Attack

[0043] An attack is carried out on a phosphate ore having a P₂O₅ contentof 36.54% and a Ca content of 38.63%. This ore is as extracted. It hasonly undergone sorting so as to eliminate at a maximum the non-phosphatewaste.

[0044] It is brought to a reactor provided with a stirrer into which anaqueous solution of phosphoric acid with 30% P₂O₅ is introduced.Digestion takes place in the reactor for a period of 15 minutes atambient temperature with the formation of a mass of attack product inthe form of pulp. During this digestion, the phosphoric acid attacks theore in order to form calcium dihydrogenophosphate [Ca(H₂PO₄)₂], which issoluble in water.

[0045] The digestion conditions are established so as to avoid anyprecipitation of calcium phosphate. These conditions are well known inthe art and depend in particular on the temperature applied and the P₂O₅concentration of the acid.

[0046] After the attack in the reactor, the pulp undergoes a separationoperation. In this example embodiment, this separation comprises firstof all a decantation of the pulp in a decanter, where it remains for aperiod of 10 minutes.

[0047] In this way a supernatant liquid containing fine grains insuspension is obtained. In this example embodiment, the liquid is thensubjected to clarification by filtration through a filter paper. Thefiltrate obtained (Sample 1) is an aqueous solution of phosphoric acidat approximately 30.5% PO₂, which contains 3.26% Ca and largely containsno impurities, such as Fe, F, Si etc. This filtrate at least partlyforms the liquid phase which, according to the invention, will besubjected to treatment with an acid.

[0048] In order to increase the yield, it is also possible to providefor washing with water of the deposit of coarse grains which hassedimented at the bottom of the decanter. In this example embodiment,four successive washings have been provided, each with water, for fiveminutes, and then a filtration. A cake of washed coarse grains isobtained (Sample 3).

[0049] The supernatant from each of the washings and the filtrate arecombined and once again filtered. In this way a cake of fine grains(Sample 2) and a liquid filtrate (Sample 4) are obtained.

[0050] The proportions of various materials of the products of thisattack were analysed and are indicated in Table I below. TABLE I ProductP₂O₅ % Ca % Fe ppm F % Si % Ore 36.54 38.63 3320 3.5 2.5 Sample 1 30.523.26 28 0.0779 — Sample 2 32.94 19.01 25064 16.39 4.55 Sample 3 26.6129.99 11906 6.99 12.8 Sample 3 1.64 0.2568 2 — —

[0051] As can easily be seen, the product Sample 1, which is the sampleof the liquid phase which will subsequently undergo treatment with anacid, now contains only traces of impurities and has a P₂O₅ content ofapproximately 30%.

EXAMPLE 2

[0052] Phosphoric Attack

[0053] It is also possible to provide a variant embodiment according tothe invention in an ore phosphoric attack installation as illustrated inthe accompanying FIG. 1.

[0054] A phosphate ore is fed at 1 into a reactor 2 at which, at 3,there are introduced water and an attack liquid containing phosphateions. A pipe 5 discharges CO₂ gas through the top of the reactor.

[0055] After digestion in the first reactor 2, the pulp formed istransferred through the pipe 6 into a first decanter 7, in which thecoarse grains of the pulp formed in the reactor 2 sediment.

[0056] The deposit of sedimented coarse grains is discharged through thebottom of the decanter at 8 in order to be transferred into a secondreactor 9 in which this deposit is subjected to an additional attack byan attack liquid containing phosphate ions. This attack liquid is herepart of the phosphoric acid solution produced according to the inventionand is fed at 10 into the reactor 9. A drain pipe 11 discharges the CO₂gas formed at the top of this reactor. The pulp formed is transferredinto a second decanter 12. The deposit in the second decanter isdischarged at 14 in the form of a cake. The supernatant from thedecanter 12 is sucked out of it at 13 and will, in this exampleembodiment, serve as an attack liquid containing phosphate ions to beintroduced into the first reactor 2.

[0057] The supernatant of the first decanter 7 is sucked into a mixer 14into which an organic phase is introduced, for example an aliphaticalcohol, in the example illustrated n-butanol. This organic phase mustpreferably be insoluble in water and lighter than water and have arepelling effect vis-à-vis the insoluble fine particles which are insuspension in the supernatant from the decanter 7. The mixture obtainedin the mixture 15 is then transferred into a clarifier 16 in which theinsoluble fine particles are concentrated at the interface between theorganic phase and the aqueous phase. This layer of insoluble fineparticles is discharged at 17 to a drying device 18, from which they aredischarged. The clarified aqueous phase, formed by an aqueous solutionof phosphoric acid containing calcium ions in solution, is discharged inthe bottom of the clarifier 16 through the pipe 19, through which it istransferred to an installation for treatment with an acid, such as theinstallation illustrated in FIG. 2.

EXAMPLE 3

[0058] Treatment with Hydrochloric Acid

[0059] The liquid issuing from the phosphoric attack and havingphosphate ions and calcium ions in solution is in this example broughtby the pipe 19, at 30, to a washing column 31 (see FIG. 2). Into thiscolumn 31 there is also introduced an organic phase based for example onbutanol, which already contains phosphoric acid (12% P₂O₅) mixed withchloride ions (4% Cl⁻).

[0060] This organic phase comes from the liquid-liquid extraction column32 and is fed to the column 31 through the pipe 33. An aqueous solutionof calcium dihydrogenophosphate leaves at the bottom of the washingcolumn 31 through the pipe 35 whilst at the top of the column an organicphase is discharged through the pipe 36.

[0061] Table II illustrates the compositions of the phases at the inletand outlet of the column 31. TABLE II P₂O₅ % Cl % Ca % Starting organicphase 10.28 4.06 0.24 solution Flow 33 aqueous phase 31.26 0 3.45 Flow30 After organic phase 11 0.21 0.1227 equilibrium Flow 36 aqueous phase26 5.25 2.86 Flow 35 Fe ppm As ppm F ppm Flow 36 17.58 <0.1 1642 Flow 35104.5 0.26 818

[0062] The column 32 contains several exchange stages providing atransfer of chloride ions present in an organic phase to an aqueousphase.

[0063] The organic phase in the column 32 consists of butanol solventand is introduced therein through the pipe 34. This organic phase haspreviously been 548% loaded with HCl through the pipe 37 in particular.The aqueous phase in the column 32 consists of an aqueous solution ofcalcium dihydrogenophosphate coming from the washing column 31 and fedthrough the pipe 35.

[0064] When the two flows pass in counter current, the phosphoric acidfrom the aqueous phase passes to the organic phase, which is fed by thepipe 33 to the column 31. The salt formed CaCl₂ on the contrary remainsin the aqueous phase, which is discharged at the bottom of the column 32through the pipe 38. This aqueous phase is deprived of its H₃PO₄ (<1% ofP₂O₅) through the effect of the liquid-liquid extraction. The flow fromthe pipe 38 is routed to a stripping reactor 39 for recovery of thetraces of solvent entrained. Steam is introduced into the reactor 39through the pipe 40. A solution of CaCl₂ is recovered at 41 and thetraces of solvent are recirculated through the conduits 51 and 52 to theorganic phase supply of the column 32.

[0065] Table III illustrates the compositions of the phases at the inletand outlet of the column 32. TABLE III P₂O₅ % Cl % Ca % Starting organicphase 0 4.8467 0 solution Flow 34 aqueous phase 19.1 5.8 2.09 Flow 35After Organic phase 11.76 4.37 0.02482 equilibrium Flow 33 Aqueous phase0.92 13.55 5.04 Flow 38 Fe ppm As ppm F ppm Flow 33 22.61 <0.1 652 Flow38 172 0.015 1392

[0066] In order to re-extract the P₂O₅ of the organic phase leavingthrough the pipe 36 of the washing column 31, the said phase isintroduced into a re-extraction column 42 into which water is introducedthrough the pipe 43.

[0067] In this way an aqueous phase of phosphoric acid is formed whichis discharged through the pipe 44. This solution can be partiallyrecycled to the phosphoric attack, for example through the pipe 10 (seealso FIG. 1). It can also be concentrated in the concentrator 45 and beused in a subsequent process or be recycled through the pipe 46 to thesupply to the column 32.

[0068] There is also discharged from this column 42, at 47, an organicphase which is taken to the mixer 48, in which fresh hydrochloric acidis fed through the pipe 49. Fresh organic solvent can be added to themixture leaving the mixer 48 through the pipe 37, for example at 50.

[0069] Table IV illustrates the compositions of the phases at the inletand outlet of the column 42. TABLE IV P₂O₅ % Cl % Starting solutionorganic phase 13.42 0.36 Flow 34 Aqueous phase 0 0 Flow 43 OutletOrganic phase 13.21 0.2464 Flow 47 Aqueous phase 27.88 0.6668 Flow 44

[0070] It must be understood that the present invention is in no waylimited to the embodiments described above and that many modificationscan be made thereto without departing from the scope of the claims whichfollow.

1. Method of producing phosphoric acid, comprising at least one attackon a phosphate ore by an attack liquid containing phosphate ions insolution, with the formation of an attack product, and a separation inthe attack product between an insoluble solid phase containingimpurities and a liquid phase presenting phosphate ions and calcium ionsin solution, characterised in that it also comprises an introductioninto the separated liquid phase of an acid which is stronger than thephosphoric acid and which forms, with the aforementioned calcium ions, asoluble calcium salt, and an isolation of a solution of this calciumsalt, not contaminated by the impurities, with the obtaining of asolution of phosphoric acid.
 2. Method according to claim 1,characterised in that the attack liquid containing phosphate ions insolution is an aqueous solution of phosphoric acid.
 3. Method accordingto claim 2, characterised in that the aqueous solution of phosphoricacid has a 20-35% P₂O₅ content.
 4. Method according to any one of claims1 to 3, characterised in that the separation comprises a settling of thesaid attack product, a drawing off of the said liquid phase in the formof supernatant, and a discharge of the said insoluble solid phase aftersedimentation.
 5. Method according to any one of claims 1 to 4,characterised in that the separated insoluble solid phase is formed froma solid phase with coarse grains which sediments, in that the separatedliquid phase having phosphate ions and calcium ions in solutioncomprises a suspension of insoluble solid phase with fine grains, and inthat the method also comprises an elimination of the fine grains insuspension by clarification in the separated liquid phase, before theintroduction of the acid.
 6. Method according to any one of claims 1 to5, characterised in that it comprises at least one additional attack ofthe said separated solid phase by an additional attack liquid containingphosphate ions in solution, with the formation of at least oneadditional attack product, and an additional separation thereof betweenan additional insoluble solid phase and an additional liquid phasecontaining phosphate ions in solution.
 7. Method according to any one ofclaims 1 to 6, characterised in that the separated liquid phase has aP₂O₅ content of 25% to 35% and a Ca content of 2% to 5%.
 8. Methodaccording to any one of claims 1 to 7, characterised in that at leastsome of the attack liquid and/or of the additional attack liquidcomprises at least some of the liquid phase resulting from the saidseparation step and/or from the said additional separation step and/orat least some of the phosphoric acid solution.
 9. Method according toone of claims 1 to 8, characterised in that the above-mentioned acid isan aqueous solution of hydrochloric acid.
 10. Method according to one ofclaims 1 to 9, characterised in that the isolation step comprises, inthe separated liquid phase treated with acid, a first extraction by anorganic solvent in order to form an organic phase containing phosphoricacid and an aqueous phase containing the Ca salt in solution and, in theorganic phase, a second extraction by water in order to form an aqueoussolution of phosphoric acid.
 11. Method according to any one of claims 1to 10, characterised in that the temperature conditions are maintainedthroughout the method between 20° and 40° C. and in that the P₂O₅concentration is maintained between 25% and 35%.
 12. Method according toany one of claims 1 to 11, characterised in that it also comprises astep of concentrating the phosphoric acid solution.